Field of the Invention
The invention relates to a method of controlling inking in a printing press by effecting specific adjustments in order to avoid scumming in the printed image.
In wet offset printing processes, a major cause of scumming is an improper proportion of the amount of dampening medium to the amount of ink applied on the printing form. If printing takes place with too little dampening medium, scumming appears in the form of tonal stripes or ink fogging or halo formation in the non-image areas of the respective printing material.
It has become known heretofore to detect these tonal stripes or the fogging optoelectronically by means of an image-detecting device directed onto the surface of the printing material. The image signals of non-image areas may be fed to a control device and used for automatically controlling the supply of the dampening medium (published European Patent Document EP 0 357 987 A1, U.S. Pat. No. 5,050,994).
Furthermore, scumming occurs especially if the temperature of the printing ink or the dampening medium is not at a certain level. In waterless offset printing, the temperature of the printing ink is especially critical (German magazine "Der Drucker", No. 17/May 6, 1993, pages W43-W44).
In order to control the influences of the temperature upon the printing ink, it has been known heretofore to provide conventional temperature-controlling devices for the dampening medium, as well as for the ink form rollers and the printing ink, respectively (published German Patent Documents DE 39 04 854 C1 and DE 40 00 912 C1).
The temperature-controlling device disclosed in the published German Patent Document DE 39 04 854 C1 is a temperature-detecting and controlling system, the function of which is to keep the temperature in the inking unit at a constant level as a function of reference data and temperature values detected in an inking unit and in the flow and return means of the temperature-controlling medium flowing through an inking roller. In each inking unit of a printing press, the reference temperature is predetermined, taking into account corrective values reflecting the properties of the dampening medium, the paper, the room temperature. the humidity in the room, and the properties of the ink. The temperature-controlling system includes a microprocessor, and the corrective values may be inputted therein via a keyboard.
A disadvantage of this procedure is that the reference temperature values and the corrective values are inputted manually and remain unchanged during the printing process. As a result thereof, the changes of the physical quantities influencing the printing process are not taken into account. For example, the room temperature, the humidity, the suction capacity of the printing material, the pH value of the dampening medium or the viscosity of the printing ink may vary, however, during the printing process. Due to the fixed corrective values, these variations would not be compensated for so that, in an extreme situation, scumming may occur, notwithstanding that the temperature of the ink form rollers is controlled.